One of the technologies for controlling air quality is the use of biofilters. Biofilters have been used for many years as a means of odor control for such installations as rendering plants and sewage treatment plants. In the past ten years, biofilters have been used as a way of controlling low concentration hydrocarbon emissions from a growing list of industries. Biofilters are getting increased attention as a possible means of dealing with low concentration hydrocarbon emissions in non-attainment areas.
Biofilters in use today are of two distinctly different types. The oldest and least efficient is the soil biofilter which is also likely the most economical to install. It consists of a soil bed which covers a distribution piping network to a depth less than three feet. Due to low soil porosity, the bed must be quite large to avoid excessive back pressure on the gas stream and to provide sufficient area to remediate the vapors. The size of these beds makes it quite expensive to place a roof over them to enable the direct measurement of emission rates. Remediation rates of one to ten kilograms per square meter of surface area per year are reported for such filter beds which are reported to operate indefinitely. The other form of biofilter is the compost filter which generally seems to be constructed of bark or sphagnum moss along with a number of different additives to improve the performance. Remediation rates with compost filters are generally reported as being between ten and one hundred grams per cubic meter of filter media per hour. While the compost beds provide a dramatic increase in performance over the soil beds, they are consumed by the remediation process and must be replenished or replaced every six months to five years. As a result of these limitations, neither type is considered particularly suitable for a variety of industrial uses such as, for example, in petroleum refining operations. Therefore, there is a need to develop still more effective biofilters and biofiltration processes for controlling air quality.